Kotlin Permutations

permutations.kt

package foo

fun <T> permutations(list: List<T>): List<List<T>> = when {
  list.size > 10 -> throw Exception("You probably dont have enough memory to keep all those permutations")
  list.size <= 1 -> listOf(list)
  else ->
    permute(list.drop(1)).map { perm ->
      (list.indices).map { i ->
        perm.subList(0, i) + list.first() + perm.drop(i)
      }
    }.flatten()
}

permutationsTest.kt

package foo

import org.amshove.kluent.shouldBeEqualTo
import org.junit.jupiter.api.Test

class PermutationTest {

  @Test
  fun `test permutations method`() {
    permute(listOf(1, 2, 3)).toSet() shouldBeEqualTo setOf(listOf(1, 2, 3), listOf(1, 3, 2), listOf(2, 1, 3), listOf(2, 3, 1), listOf(3, 1, 2), listOf(3, 2, 1))
  }

}